In the manufacture of semiconductor devices and integrated circuits it is known to modify the semiconductor substrate material by diffusing or implanting atoms or molecules of selected dopants into the substrate. The substrate materials are usually silicon or germanium.
This process produces regions of selected varying conductivity in the semiconductor substrate, with majority charge carriers of different polarities. Typical dopant materials used are boron, phosphorus, arsenic and antimony.
Doping the semiconductor substrate using ion implantation has become increasingly important with the continuing reduction in feature sizes on integrated circuit structures.
Ion implanters generally comprise a source of ions of the element to be implanted in the semiconductor substrate, and an extraction assembly for extracting the ions from the source and forming a beam of extracted ions.
The ions in the ion beam are directed towards a mass analyser and selector for selecting a particular species of ion in the ion beam. Selection is carried out using a magnetic field which is transverse to the initial direction of motion of the ions entering the mass analyser. The ion beam is deflected along a curved trajectory by the magnetic field with the radius of curvature determined by the mass/charge ratio of each ion. The exit of the mass analyser contains a slit so that only ions of a chosen mass/charge ratio are allowed to pass out of it. Any ions not having the desired mass/charge ratio impinge upon points adjacent the slit and do not exit the mass analyser, which acts as a Faraday cage.
Following mass selection, the selected ions pass along a generally linear path towards the substrate. Under certain circumstances, it may be desirable to alter the energy of the ions post mass selection, to adjust the depth to which the ions are implanted. Furthermore, co ensure homogeneous doping, it is common to scan the substrate relative to the ion beam. In one process treatment, therefore, the dose at any position on the substrate is made up of components from a large number of individual scans. Although the ion beam may be scanned relative to a fixed substrate, it is at present more usual to scan the substrate holder mechanically relative to a fixed or moving direction ion beam.
The ion beam extracted from the ion source tends to expand radially as it travels towards the mass analyser, as a consequence of, for example, space charge, Brownian (thermal) motion and the partial defocusing effect of the ion source extraction electrodes. In addition, the ion beam axis is often not initially parallel to that specified in the magnet design. The beam is required to enter the magnet at a specific angle to the pole faces if correct focussing is to be obtained. Again, the usual cause of this is mechanical misalignment between the ion source and the mass analyser, predominantly caused by deflection from the source magnetic field.
Because each of the ions of a given mass/charge ratio are moving in a fixed, transverse magnetic field, they are focussed upon the exit slit, regardless of their angle of incidence into the analyser. However, only chose ions entering the mass analyser at the specified angle will leave the exist slit with the correct trajectories. If the ion beam axis is misaligned with the entrance aperture of the mass analyser, then although each of the ions will focus upon the exit slit, they will be incident at an incorrect angle. Similarly, even if the ion beam axis (in the direction of the beam) is at the specified angle to the pole face, there will still be some ions in the (expanding) beam travelling non-parallel to that beam axis. Thus, any misalignment and/or divergence in the ion beam extracted from the ion source will translate into a respective misalignment and/or divergence in the ion beam post mass selection.
Such aspects of the ion beam are undesirable. The ion implanter usually contains a number of electrodes downstream of the mass analyser, for example to accelerate or decelerate the ions prior to implantation. These electrodes are typically coaxial with the axis of the ion beam, and thus divergence in or misalignment of the beam causes the ions to strike the electrodes. This causes sputtering of the electrodes, which introduces unwanted ions into the implanter, and also causes a part of the ion beam to be wasted. Thus, attempts have been made to address both beam divergence (defocusing) and beam direction.
The width and direction of the ion beam could be adjusted post mass selection. The problems with this are the added complexity of additional moving parts, implanter contamination and an increase in beam path length.
As an alternative, the ion beam direction may be adjusted prior to mass selection. To minimise the solid angle of the ion beam as it enters the mass analyser (and hence minimise the beam's solid angle upon exit therefrom), the entrance aperture of the mass analyser is fitted with a baffle. This baffle has a hole in it with a diameter which allows a central part of the ion beam to be passed whilst absorbing the radial periphery of the beam.
To adjust the angle of incidence of the ion beam from the ion source, relative to the entrance aperture of the mass analyser, (a process often called “tuning” of the ion beam), the electrodes in the ion source have traditionally been mechanically adjustable. By moving the electrodes in both a plane parallel to, and a plane perpendicular to, the entrance aperture of the mass analyser, the angle of the ion beam axis may be adjusted. This tuning process is carried out prior to commencement of implantation of a substrate and largely involves trial and error; an operator moves the ion source (extraction) electrodes are moved until the size and shape of the ion beam at the substrate holder is optimised. Scanning and focussing of the beam is a time consuming operation.
Commonly assigned U.S. patent application Ser. No. 08/962,257, entitled “METHOD AND APPARATUS FOR ION BEAM SCANNING IN AN ION IMPLANTER”, to England and Holmes, the content of which is incorporated herein by reference in its entirety, describes a tetrode ion extraction assembly for scanning the ion beam at the ion source. One of the four electrodes is split, and to each half of this electrode is applied an AC voltage which is independently adjustable. This technique permits electrostatic steering of the ion beam and thus obviates the need for mechanical adjustment of the electrodes co align the ion beam. However, in the technique disclosed in U.S. Ser. No. 08/962,257, a sawtooth voltage is applied to the split electrode such that the ion beam axis scans repeatedly about a mean angle, in turn to scan the post mass selection ion beam across the substrate.